1. Technical Field of the Invention
The present invention relates in general to the mobile radio communications field and, in particular, to a method and system for improving handoffs of mobile radio terminals between radio base stations.
2. Description of Related Art
In cellular mobile radio systems, the mobile radio terminals (mobile stations or MSs) are allowed to move from one cell to another. Consequently, there is a need to be able to handoff these MSs from the radio base station (BS) of the one cell to the radio BS of the other cell (inter-cell handoff). When a handoff of a MS is to be performed from a serving BS to a target BS, the target BS selection is typically made based on the following three criteria (in descending order of importance): (1) information about the expected increased quality of the connection being considered after the handoff (xe2x80x9cown interestxe2x80x9d aspect); (2) information about the expected decreased quality of other connections (xe2x80x9ccourtesyxe2x80x9d aspect); and (3) information about the traffic load in the target BSs being considered (xe2x80x9cavailabilityxe2x80x9d aspect).
In this regard, (directional) antenna arrays or xe2x80x9csmart antennasxe2x80x9d have been introduced into the radio BSs of cellular mobile radio systems, in order to direct narrow transmitting and receiving beams (lobes) from the BSs more precisely toward the MSs being served. The use of these directional narrow beam antennas improves the carrier-to-interference ratio (C/I) in the cellular system, and also increases the system""s traffic handling capacity. As such, a need has arisen to provide an effective handoff technique that takes advantage of the benefits afforded to cellular systems including radio BSs with directional antenna arrays.
PCT Application No. PCT/SE95/00764 (WO 96/00484) discloses an intra-cell (within a cell) handoff method based on the use of spatial information measured with an antenna array. Swedish Patent Application No. 9702597-7 (P08214) discloses an intra-cell handoff method used in a radio BS including an array antenna. With these approaches, information about the Direction-of-Arrival (DOA) information used for transmissions from the BS before the handoff occurs, is made available by the serving transmitter-receiver to the target transmitter-receiver for use after the handoff occurs. However, a common drawback of these conventional approaches is that they both deal specifically with intra-cell handoffs. Consequently, an improved method is needed for handling inter-cell handoffs when antenna arrays or xe2x80x9csmart antennasxe2x80x9d are being used.
The use of array antennas in cellular network radio BSs creates a need during handoffs to select an optimal target radio BS from among a plurality of radio BSs. These radio BSs can utilize omni-directional-antennas, sectored antennas, and array antennas. There is also a concomitant need to be able to select a traffic channel in the target BS.
As such, a problem addressed by the present invention is how to provide measurements for use by a handoff algorithm when a system includes BSs with array antennas. Another problem addressed by the present invention is how to use narrow beams in a target BS immediately after an inter-cell handoff occurs.
It is, therefore, an object of the present invention to provide a method and system for determining well in advance of an inter-cell handoff, the path loss and direction of a narrow beam that will be used for communications after the inter-cell handoff occurs, wherein a target BS for the handoff of a MS is selected from among at least two candidate target BSs, and at least one of candidate target BSs has an array antenna. The resulting measurement information derived can be used in a handoff algorithm to select the target BS and the traffic channel to be used.
It is another object of the present invention to provide a method and system for determining well in advance of an inter-cell handoff, the path loss and direction of a narrow beam that will be used for communications after the inter-cell handoff occurs, wherein the target BS for a handoff of a MS is selected from among at least two candidate target BSs, at least one of the candidate target BSs has an array antenna, and also for determining well in advance (but with a relatively simple technique) the quality of a connection to be expected on a narrow lobe traffic channel in the target BS having the array antenna. The resulting supplementary measurement information can be used in a handoff algorithm to select the target BS and the traffic channel to be used. (Note: Path loss is equal to transmitted signal strength minus received signal strength. Since the MS""s transmitted signal strength is known to the system, the term xe2x80x9cpath lossxe2x80x9d is not referred to hereinafter and instead in the BS received signal strength is used.)
It is still another object of the present invention to provide a method and system that enables the use of a narrow beam traffic channel in the target BS immediately after an inter-cell handoff has occurred.
The foregoing and other objects are achieved by a method and system for improved inter-cell handoffs, wherein at least one target BS including an array antenna is equipped with a Modified Receiver Module (ModRX). The ModRX is ordered to perform measurements on a MS served by a neighbor BS. The measurements include at least DOA information and uplink received signal strength (SS-ur) information. The ModRX reports the measurement information to the network controller, which can decide to handoff the MS to a specific traffic channel in a target BS based in part on the measurement information reported by the ModRX.
In one embodiment of the present invention, the DOA and SS-ur measurement information are supplemented by quality measurements made in the ModRX of the target BSs with respect to one or more idle (candidate BSsxe2x80x2) traffic channels having the same DOA as the measured DOA of the MS to be handed off (e.g., a common lobe). For example, these quality measurements preferably comprise the uplink received signal strength of the interference (SS-ur-i) in the idle candidate BSsxe2x80x2 traffic channels. The SS-ur from the MS, and the SS-ur-i of the idle candidate(s) are then used together with the known uplink transmitted signal strength (SS-ut) of the MS as an input to the handoff algorithm to be used for selecting a target BS and traffic channel.
In a second embodiment of the present invention, the DOA measured and reported by the ModRX is used by the network controller after a handoff decision has been made, to order the target BS to transmit in the direction (from the DOA information) reported by the ModRX.
An important technical advantage of the present invention is that by reporting the DOA information about the MS to be handed off, and the interference of the possible narrow lobe target traffic channels in the same DOA, the network controller can use a simple handoff algorithm to choose between narrow lobe and wide lobe traffic channel alternatives for all types of BSs using comparable criteria, such as, for example, the C/I.
Another important technical advantage of the present invention is that the handoff method that can be used allows the use of a narrow lobe immediately after the handoff, rather than the conventional approach that first uses a wide lobe in the target BS, finds the (new) DOA in the target BS, and then uses a narrow lobe.
Still another important technical advantage of the present invention is that the handoff method used identifies the best narrow lobe DOA information from the MS to be handed off to the target BS, even if a reflected beam is the best beam to use.
Yet another important technical advantage of the present invention is that certain popular mobile communications systems, such as, for example, the Ericsson CMS 88 mobile communications system includes modules in the BSs that are capable of measuring SS-ur while performing a xe2x80x9cverificationxe2x80x9d before handoff (i.e., checking SS-ur in the target BS). As such, the specialized ModRX can be a modification of an existing measurement module that, for example, adds a DOA measurement device to an existing measurement module. Consequently, the procedures and signal protocols necessary to perform the DOA and other measurements are already available in the existing module.